背弧オフィオライトの形成モデル 奥尻海溝（日本海）の例

Abstract

Two possible examples of active back-arc ophiolite emplacement can be demonstrated through some multichannel reflection seismic profiles obtained from the Japan Sea. Lithospheric convergence has been suggested along the eastern margin of the Japan Basin. Especially, some interesting structures are seen around the Okushiri Ridge.(1) Subduction with back-thrusting type: Along the 44° line in latitude, the northern part of the Okushiri Ridge is detached from the oceanic crust of Japan Basin by two thrust zones having different polarities and ages. The main boundary between the Japan Basin and the Okushiri Ridge coincides with an thrust plane (approximately 2-3° dip) with the deformation of lower sedimentary unit (Middle Miocene to Pliocene). The boundary between the Okushiri Ridge and the Musashi Basin, however, is represented by active back-thrustings against the main thrust of counter boundary between the Japan Basin and the Okushiri Ridge. The formation of the back-thrustings may relate to the subduction of the Japan Basin.(2) Obduction type: In the middle to southern part of the Okushiri Ridge, the basement, oceanic crust layer H, is continuously traceable down to the Japan Basin. Active thrusts are observed in the boundary between the Okushiri Ridge and the Shiribeshi Trough, which dip to the west. The sediments of the Shiribeshi Trough are disturbed and faulted with listric reverse faults because of the obduction of oceanic crust, the Japan Basin, against to the island arc. And also some back-thrustings develop around the boundary between the Japan Basin and the Okushiri Ridge against to the obduction. It may relate to the degree of convergence of the main detachment thrust.This lithospheric convergence may initiate from Middle Miocene, it is just after the spreading of the Japan Basin under the compressive regime. These two styles of lithospheric detachment may be an indication of nascent back-arc closing. Back-thrustings are very important structure for the detachment of pre-ophiolite sequences. Moreover, the continuation of such back-arc closing should result in an ophiolite emplacement in a short time. It is suggested that an embryo of ophiolite complex, the Okushiri Ridge, is about to be emplaced on the Japanese arc.